1. Field of the Invention
This invention relates to a photomask used in photolithography, and more particularly to an improvement in the photomask used in photolithography for production of LSI semiconductor device.
2. Description of the Prior Art
Referring to FIG. 7, there is shown, in plan view, an example of a conventional photomask. In the figure, a light-shielding layer 61 of chromium (Cr) is provided on a light-transmitting substrate 60 such as a glass plate. The light-shielding layer 61 is provided with an aperture 61a, which is substantially square. Light passing through the aperture 61a is projected onto a semiconductor wafer through a lens system.
When the photomask shown in FIG. 7 is exposed to incident light having a uniform intensity distribution with an initial intensity of "1", the transmitted light directly below the aperture 61a has an intensity of "1", whereas the transmitted light directly below the light-shielding layer 61 has an intensity of "0". That is to say, the transmitted light has a stepped form intensity distribution, with an intensity level of "1" or "0", in the vicinity of the aperture 61a. As the length of one side of the aperture 61a approaches the limit of resolution limit, however, the transmitted light projected onto the wafer through the lens system comes to have an intensity distribution represented by the concentric circular contour lines in FIG. 8A, because of diffraction caused by the small hole.
FIG. 8B shows the intensity distribution of light along line 8B--8B of FIG. 8A. It is seen that, in spite of the substantially square shape of the aperture 61a formed in the photomask shown in FIG. 7, the pattern of the light project on the wafer is substantially circular and, further, the intensity distribution of the projected light is not a stepped distribution but a Gaussian distribution.
Referring now to FIG. 9, there is illustrated the shape of a hole formed in a resist layer by use of the photomask of FIG. 7. The resist layer 91 is provided on a semiconductor wafer 90, and the hole 92 is formed in the resist layer 91 developed. In the perspective view in FIG. 9, the wafer 90 and the resist layer 91 are depicted as cut along a vertical plane passing through the center of the hole 92, in order to show the shape of the hole 92 more clearly. As is apparent from the figure, the hole 92 has the shape of a cylindrical well. That is, as the size of the hole to be formed in a resist layer becomes smaller, it becomes difficult to form a hole with a substantially tetragonal shape in horizontal section by use of a photomask according to the prior art.
As mentioned above, with the conventional photomasks it is difficult to focus a tetragonal optical pattern onto a wafer with good fidelity. In addition, the conventional photomasks have the problem of deterioration in the contrast of the optical pattern.